fix(security): anti-replay, 0600 client key, open redirect, supply-chain

Address findings from a repo security review:

- core: add a sliding-window anti-replay filter over the AEAD-authenticated
  sequence in Session (poll_input/poll_frame), closing the input-replay gap the
  data plane previously left to the LAN/VPN trust assumption. 4096-deep window,
  unit-tested; the encrypted loopback suite confirms no false drops.
- clients: write the mTLS client private key 0600 and lock the config dir 0700
  on Unix (it was world-readable at the umask default), re-locking existing
  stores on load. pf-client-core::trust plus the probe's own identity writer.
  Windows keeps the %APPDATA% ACL; Android/Apple already wrap the key.
- web: fix a post-login open redirect — resolve `next` via URL and require it to
  stay same-origin, rejecting `/\evil.com` and tab/encoding variants the old
  `!startsWith("//")` guard missed. Also fixes the dead safeNextPath helper.
- ci: SHA-256-pin the BtbN FFmpeg DLLs bundled into the signed Windows installer
  (were fetched from the rolling `latest` tag unverified); fails closed on a
  re-roll, matching the VB-CABLE gate.
- ci: fail-open fork-guard on the Windows/Apple host-mode PR build jobs that
  share runner labels with the signing jobs. Definitive fix stays server-side
  (Gitea outside-collaborator approval / isolated PR runners) — see the notes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/punktfunk-core/src/session.rs

@@ -28,15 +28,18 @@ pub struct Frame {
 /// One end of a stream. Constructed for a single [`Role`]; calling the other role's
 /// methods returns [`PunktfunkError::InvalidArg`].
 ///
-/// Note: the AEAD layer authenticates each datagram but does **not** provide anti-replay.
-/// Video replays are largely absorbed by the reassembler's per-frame dedup, but replayed
-/// input events are not yet filtered. A sliding-window replay filter keyed on the
-/// authenticated sequence belongs with the pairing/handshake layer (the GameStream host); until then,
-/// rely on the LAN/VPN transport assumption (plan §1).
+/// Anti-replay: the receive path runs each opened datagram's AEAD-authenticated sequence through a
+/// sliding-window filter ([`ReplayWindow`]), so a captured, validly-sealed datagram can't be replayed
+/// by an on-path attacker — closing the input-replay gap that previously rested solely on the
+/// LAN/VPN transport assumption (plan §1). Genuine reordering within the window is still accepted;
+/// video additionally benefits from the reassembler's per-frame dedup.
 pub struct Session {
     config: Config,
     coder: Box<dyn ErasureCoder>,
     crypto: Option<SessionCrypto>,
+    /// Anti-replay window over the peer's authenticated sequence (receive side). `Some` exactly when
+    /// `crypto` is — the plaintext probe path carries no sequence to filter on.
+    replay: Option<ReplayWindow>,
     transport: Box<dyn Transport>,
     packetizer: Packetizer,
     reassembler: Reassembler,
@@ -68,11 +71,15 @@ impl Session {
         let crypto = config
             .encrypt
             .then(|| SessionCrypto::new(&config.key, config.salt, config.role));
+        // A receive-side replay window exists exactly when the datagrams are sealed (they carry the
+        // authenticated sequence the window keys on). Both roles receive from their peer.
+        let replay = config.encrypt.then(ReplayWindow::new);
         let packetizer = Packetizer::new(&config);
         let reassembler = Reassembler::new(ReassemblerLimits::from_config(&config));
         Ok(Session {
             coder,
             crypto,
+            replay,
             transport,
             packetizer,
             reassembler,
@@ -130,6 +137,16 @@ impl Session {
         }
     }
 
+    /// Feed an opened datagram's authenticated sequence to the anti-replay window: `true` = fresh
+    /// (accept), `false` = a replay or older than the window (drop). Returns `true` when the session
+    /// isn't encrypting (no window, and no sequence on the wire to key on).
+    fn accept_seq(&mut self, seq: u64) -> bool {
+        match self.replay.as_mut() {
+            Some(w) => w.accept(seq),
+            None => true,
+        }
+    }
+
     // -- Host path --------------------------------------------------------
 
     /// Host: FEC-protect, packetize, and seal one encoded access unit into wire packets WITHOUT
@@ -225,6 +242,13 @@ impl Session {
                 Ok(p) => p,
                 Err(_) => continue, // drop undecryptable noise
             };
+            // Anti-replay: a captured input datagram replayed by an on-path attacker opens cleanly
+            // (its sequence + tag are still valid) — the window is what rejects the second copy.
+            // `len >= 8` is guaranteed because the sealed-path open above succeeded.
+            if self.replay.is_some() && !self.accept_seq(seq_of(&wire)) {
+                StatsCounters::add(&self.stats.packets_dropped, 1);
+                continue;
+            }
             StatsCounters::add(&self.stats.packets_received, 1);
             if let Some(ev) = InputEvent::decode(&pkt) {
                 return Ok(Some(ev));
@@ -276,6 +300,13 @@ impl Session {
                 Ok(p) => p,
                 Err(_) => continue,
             };
+            // Anti-replay (same rationale as poll_input): reject a datagram whose authenticated
+            // sequence was already seen. Video also dedups per-frame downstream, but filtering here
+            // is uniform and cheap. `len >= 8` because the sealed-path open above succeeded.
+            if self.replay.is_some() && !self.accept_seq(seq_of(&self.recv_scratch[i][..len])) {
+                StatsCounters::add(&self.stats.packets_dropped, 1);
+                continue;
+            }
             StatsCounters::add(&self.stats.packets_received, 1);
             StatsCounters::add(&self.stats.bytes_received, pkt.len() as u64);
             // The reassembler validates the packet via its parsed header (`magic`),
@@ -347,3 +378,162 @@ impl Session {
         Ok(())
     }
 }
+
+/// Extract the AEAD-authenticated 8-byte big-endian sequence prefix from a sealed wire datagram.
+/// Only called on the encrypted receive path, where a preceding successful open has already
+/// established `wire.len() >= 8`.
+fn seq_of(wire: &[u8]) -> u64 {
+    u64::from_be_bytes(wire[..8].try_into().unwrap())
+}
+
+/// Depth of the anti-replay window, in sequences. The sender advances its sequence once per
+/// datagram, so at the data plane's packet rate 4096 is roughly 33 ms of reorder tolerance for the
+/// video stream (well beyond any reordering still useful for a live frame) and effectively unbounded
+/// for the sparse input stream — while bounding how far back a replay could hide.
+const REPLAY_WINDOW: u64 = 4096;
+const REPLAY_WORDS: usize = (REPLAY_WINDOW / 64) as usize;
+
+/// Sliding-window anti-replay filter over the AEAD-authenticated wire sequence. The sender counts
+/// its datagrams from 0, and the protocol never legitimately re-sends a sequence (FEC recovery
+/// shards get fresh ones), so a sequence seen twice is a replay. The AEAD tag already authenticates
+/// the sequence — a forged one can't open — so this only has to reject *duplicates* of validly
+/// sealed datagrams (and anything older than the window, which we can no longer prove is fresh).
+/// Genuine reordering within the window is accepted. Bitmap-per-sequence, indexed `seq % WINDOW`.
+struct ReplayWindow {
+    /// Highest sequence accepted so far; `seen` stays false until the first datagram.
+    highest: u64,
+    seen: bool,
+    /// One bit per in-window sequence in `(highest - WINDOW, highest]`.
+    bits: [u64; REPLAY_WORDS],
+}
+
+impl ReplayWindow {
+    fn new() -> ReplayWindow {
+        ReplayWindow {
+            highest: 0,
+            seen: false,
+            bits: [0; REPLAY_WORDS],
+        }
+    }
+
+    #[inline]
+    fn word_bit(seq: u64) -> (usize, u64) {
+        let idx = (seq % REPLAY_WINDOW) as usize;
+        (idx / 64, 1u64 << (idx % 64))
+    }
+    fn is_set(&self, seq: u64) -> bool {
+        let (w, b) = Self::word_bit(seq);
+        self.bits[w] & b != 0
+    }
+    fn set(&mut self, seq: u64) {
+        let (w, b) = Self::word_bit(seq);
+        self.bits[w] |= b;
+    }
+    fn unset(&mut self, seq: u64) {
+        let (w, b) = Self::word_bit(seq);
+        self.bits[w] &= !b;
+    }
+
+    /// Record `seq`, returning `true` if it's fresh (accept) or `false` if it's a replay / too old.
+    fn accept(&mut self, seq: u64) -> bool {
+        if !self.seen {
+            self.seen = true;
+            self.highest = seq;
+            self.set(seq);
+            return true;
+        }
+        if seq > self.highest {
+            // Advance the window. Sequences between the old and new high slide in unseen, so clear
+            // their (possibly stale, from a full window ago) slots — unless we jumped an entire
+            // window, in which case wipe the bitmap wholesale.
+            if seq - self.highest >= REPLAY_WINDOW {
+                self.bits = [0; REPLAY_WORDS];
+            } else {
+                let mut s = self.highest + 1;
+                while s < seq {
+                    self.unset(s);
+                    s += 1;
+                }
+            }
+            self.highest = seq;
+            self.set(seq);
+            true
+        } else if self.highest - seq >= REPLAY_WINDOW || self.is_set(seq) {
+            // Older than the window (can't prove it isn't a replay) or already seen (a duplicate) —
+            // either way, drop it.
+            false
+        } else {
+            self.set(seq); // in-window and not yet seen — a genuine reorder
+            true
+        }
+    }
+}
+
+#[cfg(test)]
+mod replay_tests {
+    use super::*;
+
+    #[test]
+    fn accepts_in_order_and_rejects_duplicates() {
+        let mut w = ReplayWindow::new();
+        for seq in 0..1000 {
+            assert!(w.accept(seq), "fresh in-order seq {seq} must be accepted");
+        }
+        // Every one of those is now a replay.
+        for seq in 0..1000 {
+            assert!(!w.accept(seq), "replayed seq {seq} must be rejected");
+        }
+    }
+
+    #[test]
+    fn accepts_reorder_within_window_once() {
+        let mut w = ReplayWindow::new();
+        assert!(w.accept(100));
+        // Earlier-but-in-window sequences (a genuine reorder) are accepted exactly once.
+        assert!(w.accept(80));
+        assert!(!w.accept(80), "second copy of a reordered seq is a replay");
+        assert!(w.accept(99));
+        assert!(!w.accept(100), "the high-water seq itself can't be replayed");
+    }
+
+    #[test]
+    fn rejects_older_than_window() {
+        let mut w = ReplayWindow::new();
+        assert!(w.accept(REPLAY_WINDOW * 2));
+        // Anything a full window or more behind the high-water mark is dropped (can't prove fresh).
+        assert!(!w.accept(REPLAY_WINDOW * 2 - REPLAY_WINDOW));
+        assert!(!w.accept(0));
+        // But just inside the window is still accepted.
+        assert!(w.accept(REPLAY_WINDOW * 2 - (REPLAY_WINDOW - 1)));
+    }
+
+    #[test]
+    fn large_forward_jump_wipes_stale_bits() {
+        let mut w = ReplayWindow::new();
+        assert!(w.accept(5));
+        // Jump far forward (more than a window). The slot for an old seq that aliases 5 mod WINDOW
+        // must read as unseen afterward, i.e. the jump cleared it — so a NEW seq there is accepted.
+        let far = 10 * REPLAY_WINDOW + 5;
+        assert!(w.accept(far));
+        assert!(!w.accept(5), "the pre-jump seq is now far older than the window");
+        // A fresh seq aliasing 5 (mod WINDOW) but inside the new window is accepted, proving the
+        // stale bit was cleared rather than mistaken for a replay.
+        assert!(w.accept(far - REPLAY_WINDOW + 1));
+    }
+
+    #[test]
+    fn first_seq_need_not_be_zero() {
+        // Startup loss can mean the first datagram we ever open isn't seq 0.
+        let mut w = ReplayWindow::new();
+        assert!(w.accept(42));
+        assert!(!w.accept(42));
+        assert!(w.accept(43));
+    }
+
+    #[test]
+    fn seq_of_reads_the_big_endian_prefix() {
+        let mut wire = 0x0102_0304_0506_0708u64.to_be_bytes().to_vec();
+        wire.extend_from_slice(b"ciphertext-and-tag");
+        assert_eq!(seq_of(&wire), 0x0102_0304_0506_0708);
+    }
+}